The present invention relates to a key telephone system which switches key telephone sets accommodated as line and extension telephone sets in a main unit (KSU).
Extension links are arranged in a conventional key telephone system to form speech paths between the extensions, thereby interconnecting the key telephone sets. However, a given key telephone set cannot be called if the line is busy. In order to solve this problem, an extension call back or extension camp on function is provided in a conventional key telephone system.
Such a function can be registered for a call if an idle extension link is present. The extension call back is performed in the following manner. After a call back is registered, an extension calling party waits in the on-hook state. When the called party hangs up the telephone, the extension calling party is called. In this case, when the extension calling party answers the phone, the called party is automatically called. The extension camp on is performed as follows. When an extension calling party waits in the off-hook state, the extension camp on is registered, and the called party is automatically called upon on-hook.
However, in a conventional key telephone system, any call is invalid and the call back or the camp on cannot be registered if an idle extension link is not present. Thus, a desired party can be called only after an idle extension link is available, resulting in inconvenience.